There is a bigger problem; however, and that problem is that a huge number of people with STDs will never have any symptoms. Because so many STDs are asymptomatic, the only way to be certain that you are not infected is to be regularly screened for STDs. How often you should be screened depends on your risk profile, but at the very least you should always consider getting screened before a new sexual partner.
So to answer the question, how do you know if you have an STD? You don't, unless you get tested. How do you know you don't have an STD? You don't, unless you're regularly screened. Symptoms such as itching, rashes, or discomfort are a good sign that something may be wrong, but even a doctor worried personally about having an STD would need to run tests on herself to be certain as to their cause. And, just as importantly, if she had no symptoms, she'd know she still needed to be tested to make certain she was fine.